1. Field of the Invention
The present invention relates to dialkyl terephthalates wherein the alkyl stands for alkyl radicals which have a longest carbon chain of at least 4 carbon atoms, to a process for preparing them, and to the use of these products.
2. Description of the Related Art
Polyvinyl chloride (PVC) is one of the most important polymers industrially. It is used in a wide variety of applications, in the form of either unplasticized (rigid) PVC or plasticized (flexible) PVC.
To produce a plasticized PVC, plasticizers are added to the PVC, and in the majority of cases use is made of phthalates, in particular di-2-ethylhexyl phthalate (DEHP), diisononyl phthalate (DINP), and diisodecyl phthalate (DIDP). As the chain length of the esters increases, the solution or gelling temperatures rise, and the processing temperatures of the plasticized PVC therefore rise. The process temperatures can be reduced again by adding what are known as fast-gellers, such as the short-chain phthalates dibutyl phthalate (DBP), diisobutyl phthalate (DIBP), benzylbutyl phthalate (BBP) or diisoheptyl phthalate (DIHP).
Owing to debates concerning reproductive toxicities, which in a number of cases have already led to increased labeling under laws which regulate hazardous substances, it must be assumed that, in the future, the use of these shorter-chain phthalates will go down significantly. There is therefore a need for plasticizers which are not subject to mandatory labeling but which can be used as fast-gellers and which are produced from raw materials available worldwide in large quantities.
Besides phthalic acid, terephthalic acid, or its dimethyl terephthalate derivative, represents a compound which, with an estimated annual production of the order of millions of tons, is available in large quantities. An example of a product mass-produced starting from terephthalic acid is polyethylene terephthalate (PET). To date, however, only one monomeric ester of terephthalic acid has acquired some significance industrially as a plasticizer for PVC, namely di-2-ethylhexyl terephthalate (DEHT or else DOTP), which is marketed by Eastman Chemical for example.
U.S. Pat. No. 5,071,690 describes the possibility of using di-n-butyl terephthalate as a plasticizer for producing polyester films. U.S. Pat. No. 6,051,305 describes the use of thermoplastic polymer particles containing dibutyl terephthalate, as toner particles.
Don Beeler in Soc. Plast. Eng., Tech. Pap. 22 (1976), 613 15 describes how terephthalic esters of alcohols having from 1 to 6 carbon atoms are with just one exception (diisopropyl terephthalate) all solids and are incompatible with PVC. Consequently, in spite of the statement that the pattern of properties of terephthalic esters corresponds essentially to that of phthalic esters longer by one carbon atom in the alcohol chain, it could not have been anticipated that terephthalic esters of alcohols having a chain length of from 4 to 6 carbon atoms might be fast-geller candidates.